In view of the vital environmental implications, there is a continuing effort to identify and clean up sites contaminated with hazardous substances, with various methods and equipment having been proposed for this purpose. In the past, one known method has been to remove the contaminated soil and dispose of it elsewhere. However, the practical and remedial shortcomings of this approach are apparent, such that various other methods and equipment have been proposed to remediate such sites by physically removing the contaminants from the soil.
Soil vapor extraction (VE) is one such technique known in the art as an effective process for the removal of volatile organic contaminants (VOCs), such as gasoline, from unsaturated subsurface soils. This process generally entails the use of one or more wells formed in the contaminated soil to be remediated, with each well being connected to a blower to draw air through the soil and vaporize the VOCs. The VOCs must then be accumulated aboveground for treatment so as not to discharge the VOCs directly into the atmosphere. A benefit of the forced air flow through the contaminated soil is that oxygen is made available to any soil and groundwater aerobic microorganisms present in the soil that are capable of biodegrading hydrocarbon contaminants in situ. In effect, ensuring an adequate oxygen supply through forced aeration of the soil, termed "bioventing" (BV), significantly enhances biodegradation in the unsaturated soil zone. However, circumstances are typically such that a majority of the contaminants present within the soil must be removed and treated aboveground as described previously.
A technique for removing contaminants from a saturated soil zone is known as dewatering (DW) and is accomplished by directly pumping groundwater from a well that extends into the saturated soil zone. Dewatering serves to remove dispersed and dissolved organic contaminants in the pumped groundwater and significantly enhances soil remediation when used in combination with both vapor extraction and bioventing processes. Injecting air beneath the soil's water table, a technique referred to as "air sparging," serves to strip dissolved VOCs from the groundwater, which can then be captured by a vapor extraction system. Sparging is often termed "biosparging (BSp)" when the additional oxygen supply enhances biodegradation in the saturated soil zone. For more effectively removing VOCs from soil, well systems have been proposed that combine these processes simultaneously--vapor extraction, bioventing, biosparging and dewatering.
Though the above techniques are widely practiced in one form or another to remediate contaminated soil, each is associated with certain complications, disadvantages and shortcomings. For example, the emission of vapor phase contaminants from a vapor extraction system presents a serious regulatory problem, and the resulting requirement for appropriate treatment equipment greatly increases the cost of site cleanup. In particular, VOCs stripped from soil by a vapor extraction system become entrained in the air flowing into one of the system's remediation wells and, because VOCs are a hazardous waste, cannot be discharged into the atmosphere under state and federal regulations. Therefore, the VOC-containing air drawn into the well must be monitored and routed to an aboveground waste treatment system, such as an activated carbon absorption system or a thermal destruction unit (TDU), where the VOCs are absorbed or burned to prevent their emission into the atmosphere. While such techniques are capable of effectively eliminating VOCs as a hazardous waste of the remediation process, considerable costs are initially incurred to buy and maintain these systems, including the replacement of the activated carbon and/or maintaining a sufficient temperature in a thermal destruction unit to burn the VOC vapors. Additional expenses are incurred for the preparation and issuance of discharge permits for vapor extraction systems. Furthermore, if an activated carbon absorption system is employed, the spent carbon presents an additional waste problem, thereby incurring significant costs for its handling and disposal.
Similar problems arise when dewatering techniques are employed to remediate contaminated soil. For example, the contaminated groundwater that accumulates in the wells must be pumped to an above ground treatment facility. As such, disadvantages noted for vapor extraction systems are also associated with dewatering systems, including the requirement for extensive treatment equipment and discharge permits for the treatment and eventual release of the treated groundwater.
To avoid or reduce the reliance on large aboveground treatment facilities necessitated by standard vapor extraction and dewatering techniques, biological remediation processes can be employed at contaminated sites to biodegrade contaminants in situ, thereby converting the contaminants within the soil to such byproducts as water and carbon dioxide. Bioremediation is a particularly attractive approach to remediating contaminated sites, because the contaminants at most cleanup sites are often volatile, biodegradable organic substances. Furthermore, water-soluble, biodegradable organic contaminants are often the cause of groundwater contamination sites. As such, bioremediation is a possible remediation technique at these sites to eliminate the offending contaminants in situ.
Though the byproducts of a bioremediation process are environmentally safe, certain environmental regulations exist that can delay and even prevent the implementation of biological remediation technologies. Most notably, permits must be obtained before microbes, especially microbes from other sites and genetically-engineered microbes, and beneficial nutrient chemicals can be released into the subsurface soil. Biodegradation of soil contaminants is possible at some sites by using native microorganisms already growing in the soil and groundwater which have adapted to degrade the contaminants. However, at many sites the contaminants or site conditions require the use of other microbes cultured in the laboratory and chemical supplements, including sources of oxygen, oxidation agents, or other electron acceptors and chemical nutrients. Furthermore, even where a beneficial microbial population is present, an adequate population may not be sustainable if predatory, competitive, or inhibitory microorganisms also exist in the contaminated soil.
From the above, it can be appreciated that in situ biological treatment of contaminated sites is a highly desirable remediation technique, but at the present is not practical or feasible under circumstances where adequate microbial activity is not already present. Accordingly, it would be desirable to provide a method for promoting bioremediation of contaminated soil, wherein enhanced microbial activity can be achieved without reliance on directly introducing microbes and nutrients into the soil to be remediated.